Answer:
a) P [X ≥ 30 ] =  0,8413      or    84,13%
b)  P [X < 24]  = 0,0228      or    2,28 %
c) P [ 24 < X < 48 ]   =  0,9544     or   95,44%
Step-by-step explanation:
z = ( X - μ₀ )/σ
μ₀ the mean ( average no. of months that an employee stay in a factory)
σ standard deviation
a) P [X ≥ 30 ] = 1 -  P [X < 30 ]
 P [X < 30 ] 
We look for z (score)
z = ( X - μ₀ )/σ      ⇒  z =  30 - 36 / 6
z = - 1
From z table we get for -1   
 P [X < 30 ] = 0,1587
And
P [X ≥ 30 ] = 1 -  P [X < 30 ]  ⇒        P [X ≥ 30 ] = 1 - 0,1587
P [X ≥ 30 ] =  0,8413      or    84,13%
b) P [X < 24] 
z (score) = ( 24 - 36 ) / 6
z( score) = -2
And from z table we get:
 P [X < 24]  = 0,0228      or    2,28 %
c) P [ 24 < X < 48 ]    is  P[X ≤ 48] -  P[X ≤ 24]
P [X < 48]
s (score) = 48 - 36 / 6      ⇒   z = 2
P [X < 48] = 0,9772 
Then
 P [ 24 < X < 48 ]   = 0,9772  - 0,0228
P [ 24 < X < 48 ]   =  0,9544     or   95,44%